A conventional turbocharger speed sensor detects a rotational speed of a turbocharger by detecting a rotational speed of a centrifugal compressor in the turbocharger, because the ambient temperature of the centrifugal compressor is relatively low.
A turbocharger speed sensor disclosed in JP-2003-240788 includes a pressure sensor. The pressure sensor detects a periodic change in pressure caused by passage of an aluminum blade of the centrifugal compressor. The turbocharger speed sensor detects the rotational speed of the compressor based on the periodic pressure change.
Another known turbocharger speed sensor detects the rotational speed of the compressor using an eddy current. In the turbocharger speed sensor, an alternating current is supplied to a coil so that the coil generates alternating magnetic field. When the blade of the compressor passes through the magnetic field, the eddy current is induced in the blade. An impedance of the coil changes due to the eddy current. The turbocharger speed sensor detects the rotational speed of the compressor based on the change in the coil impedance. This type of sensor is called as an eddy current type sensor and disclosed in JP-2000-121655.
Since the pressure sensor is generally a semiconductor device, accuracy of the pressure sensor changes with a change in temperature. Further, although the ambient temperature of the centrifugal compressor is relatively low, the ambient temperature may exceed maximum operating temperature of the pressure sensor. Therefore, accuracy of the turbocharger speed sensor disclosed in JP-2003-240788 may decrease under such a high temperature environment.
The eddy current type sensor is suitable for use in the turbocharger speed sensor because the maximum operating temperature of the coil is greater than that of the pressure sensor. However, since the coil impedance changes with the temperature change, accuracy of the eddy current type sensor may decrease under such a high temperature environment. Further, since the coil impedance slightly changes with the change in the eddy current, the change in the coil impedance due to the temperature change greatly affects the accuracy of the eddy current type sensor.